Chloroperoxidase (CPO), a heme protein isolated from the mold Caldariomyces fumago, reacts in the presence of H2O2 with many alkenes to form epoxides. Terminal alkenes, however, inactivate the enzyme forming a green N-alkyl derivative that reverts partially to an active form of CPO. We have followed the reaction by EPR and found the green N-alkyl derivative to be a single low-spin species with {{g = (1.95, 2.16, 2.32), reverting to native CPO with {{g = (1.84, 2.26, 2.62), and a transient quasi-native species with {{g = (1.88, 2.26, 2.51).